1. Technical Field
The present invention relates generally to pressure vessels. More particularly, the present invention relates to a method for the manufacture of very high pressure vessels so that they survive high cycle fatigue loading.
2. Related Art
Vessels used to contain high pressures (greater than 10,000 pounds per square inch [psi]) have been used for many years in many industries such as: cannons and small arms, materials processing such as processing polyethylene, and high pressure water jet cutting. Most high pressure commercial applications use vessels that operate at pressures no greater than about 60,000 psi. Some of these vessels, such as those used in polyethylene in polyethylene processing and water jet cutting, are subjected to high cycle fatigue loading. However, some weapons and metals processing operating pressures are as high as, and even greater than, 100,000 psi. This later group of high pressure vessels are not usually subjected to high cycle fatigue loading.
Recently, the use of pressures as high as 100,000 psi have been suggested for sterilizing certain foods. There are many advantages to using high pressure to process food since it can be accomplished at room temperature which allows the processor to control food flavor and quality better than if the food is heated such as in pasteurization. Other unexpected benefits sometimes occur when the high pressure activates some biochemical flavor enhancement reactions. However, to be economically feasible, mass quantities of food must be processed thus requiring pressure vessels that can survive high cycle fatigue loading at very high pressures. Therefore, there is a need to develop high pressure processing equipment that can survive high cycles of very high pressures (above 60,000 psi).
Fatigue failure is a progressive mode of failure that occurs when stresses or strains that will not cause failure in a single application are applied repeatedly. The failure proceeds in three stages. There is, first, a fatigue crack initiation that occurs microscopically, followed by some stable crack propagation or growth until the crack obtains a sufficient size such that the structure ruptures.
To survive a high number of cycles (greater than about 100,000 cycles) means must be taken to: 1) retard or prevent the initiation of fatigue cracks, and/or 2) retard or prevent the propagation of the fatigue cracks produced. However, the stresses produced in high pressure vessels when very high pressures are applied are such that fatigue crack initiation cannot be prevented. Accordingly, the approach of the present invention is to prevent fatigue crack propagation.
Cracks can propagate in fatigue by several mechanisms and under the influence of several loading modes. However, the most damaging and fastest crack growth occurs when the crack is propagated in a direction perpendicular to an applied tensile stress. This is the opening mode of crack loading/propagation. Thus a need exists to provide a method to manufacture a pressure vessel that when followed will result in very long fatigue lives by the prevention of the opening mode of crack loading.